There's no fresh news in here; this is just me wanting to discuss 5e damage types.

Damage types:

Bludgeoning / Piercing / Slashing from a magical source: These come from magic weapons, and magical weapons aren't supposed to be resisted hardly ever. Spells that deal these damage types should do so only in a limited fashion- they shouldn't be on top for good sources of damage. A fistball that deals magical bludgeoning damage to an area with a 3rd level spell would probably need to be something like 5d6, but is best avoided entirely. The ability of the Order of Scribes wizard to deal this kind of damage via technicalities was a moderate part of why I didn't include it.

Force: This doesn't represent physical force, it represents raw magical energy. It's the best damage type a spell can do. The few monsters that resist or are immune to it are thematically in-line with it; you don't resist force just because you're a real big badass guy. Amethyst Dragons and Amethyst Dragonborn take less damage from (and actively cause) force damage. It's tough to deal this damage over an area in any method. One thing that will never deal force damage over an area is an explosion.

Radiant: This is holy damage. However, it appears that radiation is also in this. Angels resist this, some evil creatures are vulnerable. This is a reliable and strong damage type, and shouldn't be given to spells that push damage boundaries directly. When 5.0 was new it seemed that this damage type wasn't something arcane casters were going to have much access to, but they did get some added. Paladins deal this with smites.

Necrotic: This is unholy evil damage and does double duty as anything that is violent physical disintegration (not including the disintegrate spell, which is force) or serious decay. There's enough powerful bad guys that resist it that it's not as valuable as radiant, but it's still effectively going to always damage whatever you point it at. I gave Dark Knights a few ways to deal this damage type, but it's not particularly uncommon on spells, with Horrid Wilting dealing it. I also added it to a weak low level druid spell that was also meant to model magically removing water from a creature to damage it. Somewhat popular immunity for anything tightly associated with negative energy.

Psychic: This is mental damage, and things that deal it don't normally cause physical harm at all. Being stabbed with a psychic dagger could kill you, but it wouldn't leave a mark. It's not as reliable as the above damage types because a few creatures resist it or are immune to it for various reasons. All objects are completely immune to psychic damage.

Thunder: From this point forward, the damage types change from being inherently magical in all cases (or almost all cases) to being material in nature. Thunder is in the middle, and this is the old sonic damage from 3.X. With a few rare cases (sound themed monsters mostly, and the silence spell) almost everything takes this damage, and it's usually fully effective against objects. As such, spells don't get to deal it freely in large amounts- shatter, for instance, is below template in part because of this damage type. Most spells that deal thunder damage also do something else with it.

Acid: While some things don't take acid damage, a lot of things that have really annoying resistances don't resist acid. For instance, acid is normally effective against fiends and corporeal undead, who have a pack of other resistances or immunities. If able to be brought to bear against objects it can do a real number on anything that is destroyed by an acid or a base (any acid-base reaction is modeled as acid damage).

Cold: A decent number of things resist or ignore cold. This is one of the core elemental damage types, and if a spell deals a pile of damage it's appropriate for it to deal cold damage. Some things in 5.X that deal cold damage demand a Constitution save even though they affect a large area and would logically need a Dexterity save; this is mechanically interesting to characters with options to deal different damage types (many tough monsters have good Con saves, some sneaky targets have evasion which provides a poor target for Dex saves).

Lightning: A classic top damage type, and a solid call for any top damaging spell. Almost nothing is ever vulnerable to it, and there's a few weird things that are empowered by it, but it's otherwise similar to fire from a game perspective, except with fewer resistances, and spells that deal damage to objects with it can melt or ignite them just as fire damage can.

Fire: If a spell is pressing boundaries for its level in terms of area and damage, it should be a fire spell (or maybe poison). Fire is common enough that resistance to it is generally the best resistance, and commonly resisted enough that a power that changes a fireball to something else is valuable and should be priced as such. It's commonly resisted and some pretty scary things are immune to fire. There's enough spells that deal fire damage that specializing in it can be smart, and this is true even after I filled out some of the other elemental aoe types. It is also worth pointing out that while fire has some creatures that resist it, it also has some that are vulnerable to it.

Poison: The weakest damage type, there's whole classes of creature that are immune, and many have wacky resistances you might not guess. I explored adding to this a little bit with some spells that deal poison damage, and of course unlike the above types it can be obtained and added to a weapon without magic.

Nonmagical Blundgeoning, Piercing, and Slashing: Mundane damage from weapons is often resisted and sometimes immuned by a variety of things. It's a damage type for low level PCs and is the only damage type that a normal mundane NPC will have round-by-round access to.

One more damage type:

To this list, I've added "Void". I did this by changing the void arms spell to deal Void damage (this isn't an important change but I obviously had to do it if I'm adding Void as a damage type), and by adding the void blade spell, based closely off the shadow blade template. It's less damage than shadow blade, which deals psychic damage.
My exotic elements exist in a ring: light beats dark, dark beats eyes, eyes beats void, and void beats light. Eyes deals psychic damage in this edition, light is a close-enough match for radiant, and dark is pretty clearly necrotic. Void is supposed to be damage type that isn't easily understood or defended against easily, so adding it is simple. The only sources are those two spells, some magic items and some custom monsters, so it's not something particularly worth caring about. None of this is changed from the spells document, where I discuss this with a DM's note on the V page, and I think I bring it up in diffs as well.

But adding a damage type notable for not having interactions is easy, most damage types that 5.X is missing DO have a lot of interactions.
For instance, say you want to make a "grass" damage type, or a "nature" damage type, or an "earth energy" damage type. In previous versions you could pick an existing damage type- for instance, "force" or "bludgeoning"- and then apply a descriptor to the spell or effect, such as [Earth]. This would interact with existing things that give bonuses or maluses to things of that type. I'm pretty sure some Wu-jen could get a bonus with [Earth] of some sort. This pretty much eliminated the need to make a new damage type, because so much of the game had stuff to plug into. If you wanted to be an earth themed wizard, a custom feat that gives you some bonus with [Earth] spells plus maybe two custom spells with that descriptor and you were well painted into the world mechanically.
In 5e, this isn't so good. Without descriptors we have to do stuff from the ground up, and generally if you wanted something to be earth damage, you'd be stuck with bludgeoning. If something were to be "grass" or "nature" damage, you're clearly looking for something next to radiant or necrotic, and nothing jumps out. Further, if you stamped one of them over it, you'd end up with really weird resistances, vulnerabilities, and immunities.

As such, adding a damage type to 5e is only easy if your damage type is meant to be rarely resisted and not the subject of a high damage spell (for instance, if an 8d6 void ball appeared as a level 3 spell, that would be way too good because only a few custom monsters or characters equipped with a rare magic item would ever resist it). Since a nature type might have such a thing (wrath of nature, solar beam), you'd need to walk whatever milieu of denizens of your world or have a type-based assignment (for instance, undead could all resist grass).

There's almost no guidance for adding a damage type to D&D 5e. It seems like a pain to add a big one and get it right, so I'm not doing that.

Other damage:

Ionizing Radiation is radiant damage. Likely there should be a damage type specifically for this, but there isn't. I may, at some point, add one, but I'd need to walk the above concerns to do so. In practice a radiation source could be argued as a mix of radiant, poison, and fire and I may model it as such should one come up.

Violent disintegration, such as caused by antimatter, is necrotic damage (an explosion caused by antimatter would not be). It's not clear why disintegrate is force damage, except that they wanted it to be like, the best damage type. If anyone has some thoughts on this I'd like to hear them because this seems like a weird thing to have in the base game. You can find the antimatter rifle (and maybe one or two other antimatter weapons) in the DMG listed as necrotic. I haven't spent much time thinking about this because frankly I'm not really expecting antimatter rifles to show up much or at all.

Explosions are are bludgeoning and/or thunder damage, but in my opinion should be their own damage type. This is the one damage type that made me start a spreadsheet and twenty minutes later I realized it would be a magnificent effort and stopped. Since explosions happen in every campaign, I'd like this to have more support from the system. Something mapping violent physical force like this should exist, and I find it conspicuous by its absence.

In other games I've had a minor damage type called "disruption", which represents a force that disrupts the existence of something physically. For something that rarely showed up all it did was cause confusion, and I've no intention of bringing it into 5e. If I were to do so, I'd bring it in as "necrotic or radiant, whichever deals more damage".

5.5 Quickchat:

In 5.0, a lot of monsters have constructions like this:
Bludgeoning, Piercing, and Slashing From Nonmagical Attacks That Aren't Silvered

In 5.5, monsters don't have this sort of thing, and abilities that used to make your stuff count as magical now turn it into force damage. This is something to keep in mind if you want to use a 5.0 monster in 5.5, or, what I actually will care about eventually, using a 5.5 monster in 5.0 (as bringing 5.5 content into a 5.0 game will remain important to me as long as I'm running 5.X- I've already completely written off running 5.5 as a baseline ever).